1. Field of the Invention
The present invention relates to toner used for copiers, laser beam printers (LBP), plain paper facsimiles, color electrophotography (PPC), color LBPs or color facsimiles and a method for producing the same.
2. Description of the Prior Art
Recently, electrophotographic apparatuses, which commonly were used in offices, have been used increasingly for personal purposes, so that technologies to achieve compact or maintenance-free electrophotographic apparatuses are required. To meet this end, it is necessary to improve maintenance properties such as recycling of waste toner and reducing emission of ozone.
It is well-know that toner for electrostatic charge development used in an electrophotographic method generally includes a resin component, a coloring component including a pigment or a dye, a plasticizer, a charge controlling agent, and an additive, if necessary, such as a releasing agent. As the resin component, natural or synthetic resin is used alone or in combination. An additive is pre-mixed in an appropriate ratio and the resulting mixture is heated and kneaded by thermal melting, and pulverized by an air stream collision board system, and fine powder is classified to complete a toner base. Thereafter, an external additive is added to the toner base externally so as to complete toner. The single component development typically uses toner only, and in the two component development system, the developer material includes toner and carrier comprising magnetic granules.
For color copiers, a photoconductive member is charged by corona discharge with a charger, and then is exposed to optical signals for latent images for each color to form electrostatic latent images. The latent images are developed by a first color toner, e.g., yellow toner, to form visible images. Thereafter, a transfer material charged with a polarity reverse to that of the charged yellow toner is contacted with the photoconductive member so that the yellow toner images formed on the photoconductive member are transferred thereto. The photoconductive member is cleaned by removing residual toner that has not been transferred, and the development and transfer of the first color toner ends with discharging the photoconductive member. Thereafter, the same operations as those for the yellow toner are repeated for toners for other colors such as magenta and cyan. The toner images of the colors are superimposed on the transfer material so as to form color images. Then, the superimposed toner images are transferred to a transfer paper charged with a polarity reverse to that of the toner, and fixed. Thus, the copying operation ends.
As a method for forming color images, a transfer drum method and a successive superimposition method generally are used. In the transfer drum method, toner images for each color are formed on a single photoconductive member one after another, and a transfer material wound on a transfer drum is opposed to the photoconductive member repeatedly by rotating the drum so that the toner images for each color formed sequentially are superimposed and transferred to the transfer material. In the successive superimposition method, a plurality of image formation sections are provided, and toner images for each color are transferred to a transfer material conveyed by a belt sequentially while moving the image formation sections so that the color images are superimposed. One example using the transfer drum method is a color image formation apparatus disclosed in Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 1-252982. One example of a color image formation apparatus using the successive transfer method is disclosed in Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 1-250970. In this conventional example, four image formation stations, each of which includes a photoconductive member, optical scanning means or the like, are arranged to form images for four colors. A paper conveyed by a belt passes below each photoconductive member so that color toner images are superimposed. As another method for forming color images by superimposing toner images for different colors on a transfer material, Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 2-212867 discloses the following method. Toner images for each color formed on a photoconductive member sequentially are superimposed on an intermediate transfer material, and then the toner images on the intermediate transfer material are transferred to a transfer paper collectively.
Further, Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 59-148067 discloses toner using as a resin an unsaturated ethylene based polymer including a low molecular weight and a high molecular weight portion, where the peak value of the low molecular weight and Mw/Mn are defined, and containing polyolefin having a specific softening point. This disclosure is intended to provide fixability and an anti-offset property. Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 56-158340 discloses toner comprising a resin including a specific low molecular weight polymer component and a specific high molecular weight polymer component as the main component. The low molecular weight component is used to provide fixability, and the high molecular weight component is used to provide the anti-offset property. Further, Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 58-223155 discloses toner having containing a resin including an unsaturated ethylene based polymer having the local maximum in molecular weight ranges of 1000 to 10000 and 200000 to 1000000 and Mw/Mn of 10 to 40, and a polyolefin having a specific softening point. The low molecular weight component is used to provide fixability, and the high molecular weight component and the polyolefin are used to provide the anti-offset property.
However, when the melt viscosity of a binding resin is reduced or a low molecular weight resin is used in order to raise the fixing strength with high-speed machines, so-called "spent", which is caused by toner adhering to carriers, may occur during long period use in the case of two component development. In the case of the single component development, the toner is likely to adhere to a doctor blade or a development sleeve, so that the stress resistance property of the toner deteriorates. When the toner is used in a low-speed machine, offset caused by toner adhering to a heat roller may occur at the time of fixing. Further, blocking caused by toner particles fused with each other may occur during long period storage.
On the other hand, the successive transfer system includes image formation positions corresponding in number to the number of colors, and a paper is allowed to pass by the image formation positions one after another. Therefore, a transfer drum is not required. However, this system requires a plurality of latent image formation means, such as laser optical systems, for forming latent images on the photoconductive member corresponding in number to the number of colors. This complicates the structure and makes the apparatus expensive. Moreover, since there are a plurality of image formation positions, positions of portions where images for different colors are formed may not match each other, the rotation axis may be off center, or the parallelism of the portions may not match. These factors prevent the colors from being placed in intended positions and make it difficult to obtain high quality images stably. In particular, it is necessary to register the latent images for different colors precisely by the latent image formation means. As shown in Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 1-250970, considerable efforts and complicated configuration for an image exposure system, which is a latent formation means, are required.
Furthermore, in the example of Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 2-212867 employing an intermediate transfer material, toner images for all colors are formed on one and the same photoconductive member. Therefore, a plurality of developing devices are required to be provided around the single photoconductive member, and therefore the photoconductive member should be large. Furthermore, the photoconductive member is belt-shaped, which is difficult to handle. Moreover, at the time of replacement of each development device for maintenance, matching adjustment is required, or at the time of replacement of the photoconductive member, position adjustment relative to each development device is required. Thus, the maintenance of the development device for each color and the photoconductive member is difficult.
Furthermore, a process for producing toner includes a pre-blending treatment, a kneading treatment, a pulverization treatment, a classification treatment and an external addition treatment. The classification treatment is intended to classify toner powder so as to obtain a predetermined particle size distribution. In the current system, the toner powder that has been classified out is disposed of, because if the removed toner powder is used to be blended again, fog increases. Especially polypropylene or polyethylene wax, which is added to improve a releasing property, increases fog significantly. The reuse of the toner powder, which is generated in an amount of about 10 to 20 wt %, would result in effective use of resources.